• Natural Killer (NK);
  • Leucocyte Ig-like Receptor Complex (LRC);
  • Natural Cytotoxicity Receptors (NCRs);
  • MHC;
  • Inhibitory Receptors;
  • Immunoglobulin Superfamily (Ig-SF);
  • Crystal Structure;
  • Activating Receptors;
  • Sialic acid-binding Ig-like lectin (Siglec)


NK cells express receptors characterized by opposite functions that finely regulate their activities. Among inhibitory receptors, some are specific for different groups of MHC class I alleles, while others are still orphan receptors. On the contrary, various activating receptors are involved in the triggering of NK-mediated natural cytotoxicity. In general, their engagement induces human NK cells to kill target cells that are either HLA class I-negative or -deficient. Thus, the process of NK cell triggering mediated by Natural Cytotoxicity Receptors can be mainly considered as a non MHC-restricted mechanism. Here, a brief description of the molecular nature of these receptors, as well as, of their 3D-structures and of the implications for ligand recognition, is given.